1. Field of the Invention
The present invention relates to styryl compounds, a process for preparing said styryl compounds and photoresist compositions comprising said styryl compounds, which photoresist compositions can be preferably used to form fine patterns on a substrate having high reflectance in the production of semiconductors.
2. Description of the Related Art
A photoresist which comprises a sensitizing compound having a quinone diazide group and a novolak resin, or which comprises a bisazide sensitizer and a cyclized rubber is used in the production of integrated circuits such as LSI.
In a process for producing the integrated circuits, fine patterns are formed on various substrates through photoresists. However, when the conventional photoresists are used on the substrates having high reflectance such as those made of aluminum, aluminum-silicon, polysilicon and the like, various problems arise. For example, a region which should not be exposed may be exposed because of reflection on a surface of the substrate and/or side walls of steps. This phenomenon is generally called as notching or halation.
To solve these problems and prevent deterioration of resolution, Japanese Patent Publication No. 37562/1976 proposes a photoresist composition which comprises, as a light absorber, a dye represented by the formula: ##STR2## having characteristic absorptions in the ultraviolet range(Oil Yellow [C.I. 11020]). This photoresist composition can decrease light transmission through the photoresist layer and reduce the undesirable exposure of the substrate.
In the context of the specification, a "photoresist" is intended to mean a composition which comprises a sensitizer and a resin such as novolak, and a "photoresist composition" is intended to mean a composition which comprises a "photoresist" and a light absorber.
In general, if the light absorber is added to the photoresist, undesirable problems may arise. For example, photoresist drastically loses its sensitivity and productivity of the semiconductors is decreased.
The photoresist layer is usually formed by applying the photoresist composition containing a solvent on a wafer and prebaking the wafer with the applied photoresist composition to evaporate off the solvent. However, some light absorbers may precipitate during storage of the photoresist composition, or sublimate during prebaking so that a concentration of the light absorber in the photoresist layer formed on the wafer may be lowered, which lead to unsatisfactory results or variation of quality of the produced semiconductors.
To solve these problems, phenylazobenzene derivatives are proposed in Japanese Patent Kokai (Laid-open) Publication Nos. 368358/1980 and 174941/1983. But, the use of such derivatives creates some problems. For example, the phenylazobenzene derivatives should be used in a large amount in order to obtain sufficient absorption at desired wavelength, especially when the prebaking temperature is raised, or such derivatives have inferior anti-sublimation, a broad absorption range, low absorbing performance and undesirable absorption at certain wavelength. Japanese Patent Kokai Publication No. 93445/1986 discloses a photoresist composition comprising, as a light absorber, a certain styryl compound. Although the disclosed styryl compound can solve the problems associated with the prebaking, it greatly decreases the sensitivity of the photoresist.